Contents

III. How to design electrodes

"Ⅰ. About electric discharge machining"

"II. How to set machining conditions"

2. How to choose a material

Choosing the electrode material depends on the type of mold.

1) How to choose the electrode material for "middle-sized or large-sized mold of plastic / die-casting"

(1) Characteristics of the mold

Molds of this field are represented by plastic molds for home electronics and automobiles. A large number of electrodes may be used and it may become around 400 of them for a single mold. In addition, the electrode sizes are widespread from a small one to a large one. Small electrodes are around 70% of the number of electrodes even for a large-sized mold, because it has a lot of small machining areas such as the reinforcement area (rib groove) above all.

  • Large number of electrodes
  • Extensive variety of electrode sizes (70% of electrodes are small ones.)
  • Emphasis on removal rate than the machining accuracy

(2) Elements required for the electrode material

The element made much of is machining efficiency at the time of the electrode production and removal rate at the time of the electric discharge machining. Graphite is used as the electrode material for this reason. Many kinds of graphite for electrodes are widely used. As for the high quality type of small graphite particle size, the finishing quality at the time of EDM is good. On the other hand, machining efficiency at the time of electrode production by milling is extremely bad. For this reason, average quality graphite of relatively large particle size is used for molds of the middle and large size. Also, graphite has an advantageous characteristic of less bending at EDM even for the thin electrode for reinforcement area.

As for a matter that requires attention on choosing graphite, an abnormal electric discharge is easy to be generated at the time of EDM, and when the electric discharge area at the time of starting EDM is small, or when EDM condition of small electrode wear is used, the risk increases in particular. For this reason, in case of using graphite, the EDM condition of a little larger electrode wear is suitable, and it is necessary to deal with it by increasing the number of electrodes for the same machining area.

  • The graphite has high machining efficiency at the time of the electrode production
  • As for the graphite, removal rate at the time of EDM is fast

2) How to choose the electrode material for "small mold of plastic / die-casting"

(1) Characteristics of the mold

Molds of this field are represented by plastic molds for such as cellular phones and digital cameras. In machining of the cavity surface, the surface finish will be required to be high quality of fine surface roughness which is easy to polish. On the other hand, in machining the core side, many small electrodes are necessary though importance of surface quality becomes small

  • High-quality surface finish cavity
  • The importance of surface quality of the core is small, but you need a large number of small electrodes

(2) Elements required for the electrode material

The important element is surface quality. For this reason, electrolytic copper is used as the electrode material. In EDM, electrode wear generally increases in the domain of fine surface roughness, but graphite has this tendency extremely much, and high quality graphite of small particle size is used for machining of small surface roughness. Still more electrode number is required than copper. On the other hand, the graphite which is high in machining efficiency at the time of electrode production is suitable for machining of core side because of a large number of electrodes.

  • Copper is suitable for high-quality machining
  • Graphite has high machining efficiency at the time of electrode production

3) How to choose the electrode material for "precision plastic molds"

(1) Characteristic of the mold

Mold of this field is represented by plastic mold for electronic parts and engineering plastics. Dimensional accuracy is required.

  • Making much of dimensional accuracy

(2) Elements required for the electrode material

The element made much of is machining accuracy. High machining accuracy is required at the time of electrode production and EDM. Generally, copper is used. And also, non-copper materials are used because of their advantages. For example, copper tungsten and graphite are used because of cutting performance at the time of electrode production and because of less thermal expansion at EDM, and graphite is used because of an advantage that an electrode is inflexible by heat at the time of EDM, and so on.

  • Copper has general appropriateness
  • Copper tungsten and graphite have an advantage of efficiency of cutting at the electrode production
  • Copper tungsten and graphite have an advantage with less thermal expansion at EDM
  • Graphite has an advantage that the electrode is inflexible by heat at EDM